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In real life, touch is not only a part of perception, but also a bridge to connect with the world. Whether it is the soft sand under our feet or the glass condensed with water droplets in our hands, the sense of touch makes us feel the texture and temperature of life. For those who have lost this ability due to damage to their nervous system, reconnecting can seem like a distant dream. However, with the continuous advancement of technology, this dream is gradually becoming a reality.
01 The revolution of biomimetic electrical stimulation
As early as the eighteenth century, it was recorded that English doctors used tribostatic electricity to stimulate the brains of patients, claiming to stimulate excitement and improve their mental state. In recent years, the "upgraded" electrical stimulation method has become an important means to restore some sensorimotor functions in people with disabilities. This method uses tiny implantable electrodes to stimulate the nervous system in an attempt to establish direct communication between the prosthesis and the brain (Raspopovic, Valle and Petrini). Still, there is still a big gap between current technology and the dexterity and tactile sensation of the natural human hand (Johansson and Flanagan). The main challenge is how to evoke intuitive and natural sensations through electrical stimulation, so that people with disabilities can better interact with external objects.
▷Figure 1: Comparison of electrical stimulation in the past and present. Source: https://www.nsmedicaldevices.com/analysis/brain-stimulation-therapy-history/ (left) and https://www.eurekalert.org/multimedia/871164 (right).
02 A breakthrough in biomimetic nerve stimulation
A groundbreaking paper in the journal Nature offers a solution to this challenge (Valle et al.). The research team has developed a neuroprosthetic framework with a biomimetic approach at its core. At the heart of this framework is a biomimetic neurostimulation model called FootSim, which is able to mimic the tactile sensation of the soles of the human foot, including subtle changes in pressure and touch. When touching a specific location on the sole of the foot, FootSim outputs a corresponding neural activation response. Researchers have devised a variety of biomimetic neurostimulation strategies using FootSim. For example, they can set up a pressure distribution that simulates a walking situation and then observe how well this distribution activates the tactile nerves on the plantar of the foot. So, is this biomimetic neurostimulation strategy unnatural?
▷Valle, Giacomo, et al. "Biomimetic computer-to-brain communication enhancing naturalistic touch sensations via peripheral nerve stimulation." Nature Communications 15.1 (2024): 1151.
03 Testing of biomimetic nerve stimulation in animals
In order to explore the question of naturalness, a key part of the research was to explore the possibility of electrical stimulation to mimic natural neural activity in cats. In the experiment, the two cats were de-cerebralized in order to analyze neural activity limited to reflex responses, while avoiding signal interferences that could be caused by voluntary movements. By implanting electrodes on the tibial nerve for electrical stimulation, the investigator was able to adjust the intensity of the stimulation slightly above the threshold for eliciting a potential response in the spinal cord. With this setup, the investigator observed and recorded neural responses in the spinal cord and dorsal root ganglia.
The results of the experiment showed that the neural dynamics generated by stimulation using the biomimetic mode were more similar to the natural neural activity generated by physical contact, such as touching a cat's leg with a cotton swab, compared to tonic stimulation. Non-natural stimulation using the conventional 50 Hz mode produced different neural dynamics. This finding validates the team's hypothesis that biomimetic stimuli can indeed more accurately simulate and replicate natural neural activity.
This animal experiment is significant in demonstrating that electrical stimulation can be used as an effective tool to generate artificial neural activation patterns similar to natural tactile perception, and that these patterns can be efficiently delivered to the upstream regions of the sensory nervous system. But what if this biomimetic stimulus were applied to humans?
04 Application of biomimetic nerve stimulation in amputation patients
Further clinical trials were conducted on three sciatic nerve amputation patients, in which peripheral nerves in the lower limbs were directly stimulated by implanted electrodes. When three participants were stimulated with biomimetic electrical stimulation, their assessment of naturalness (0 being completely unnatural, 5 being completely natural, such as the feeling of sunken skin) was around 3, while naturalness dropped to about 1 when exposed to conventional linear or sinusoidal (unnatural) electrical stimulation. This result not only validates the effectiveness of biomimetic neurostimulation coding, but also shows that it produces more natural sensations than traditional non-biomimetic stimulation methods.
The study then explored the effects of applying biomimetic neurostimulation to real-time neuroprosthetic systems. The system, which includes inductive insoles and microprocessor-based knee prostheses, converts the wearer's pressure information while walking into a bionic neurostimulation pattern. The results showed that compared with traditional neurostimulation methods, the system not only improved the patient's walking speed, but also enhanced the patient's confidence when walking up stairs and reduced the psychological burden when performing dual tasks.
▷Figure 2: Three key steps in developing a neuroprosthesis that mimics human sensation: First, a biomimetic stimulation model of the soles of the feet was used to simulate how the nervous system processes the sense of touch. Then, experiments were conducted on animals to test whether these biomimetic stimuli were similar to natural stimuli. Finally, this biomimetic stimulation technology is implanted into the human body and its effects are tested in a real-life environment. Source: Original paper.
05 The future of biomimetic neurostimulation
This study not only demonstrates the great potential of biomimetic neurostimulation to restore and enhance tactile perception, but also highlights its potential to promote more natural interactions between humans and machines. By simulating the complexities of natural touch, research offers a glimmer of hope for those who have lost the ability to sense nature, envisioning a future where the human sensory experience can be rebuilt and even enhanced through advanced technology.
As biomimetic technology continues to evolve and improve, we can expect more solutions for different sensory disability, not just touch. The development of these technologies will greatly improve the quality of life of people with disabilities, enabling them to participate in society with greater confidence and independence. These advances herald a future in which major breakthroughs will be achieved in a variety of fields, including human-computer interaction, neuroscience, and rehabilitation medicine. As these technologies mature and become ubiquitous, we can expect to witness a more inclusive and compassionate society in which everyone can fully experience and enjoy every tactile moment of life.
Bibliography:
- Johansson, R. S., and J. R. Flanagan. "Coding and Use of Tactile Signals from the Fingertips in Object Manipulation Tasks." Nat Rev Neurosci 10.5 (2009): 345-59. Print.
- Raspopovic, S., G. Valle, and F. M. Petrini. "Sensory Feedback for Limb Prostheses in Amputees." Nat Mater 20.7 (2021): 925-39. Print.
- Valle, G., et al. "Biomimetic Computer-to-Brain Communication Enhancing Naturalistic Touch Sensations Via Peripheral Nerve Stimulation." Nat Commun 15.1 (2024): 1151. Print.